(1) Field of the Invention
The present invention relates to a computer system to which display controllers of a plurality of types, such as those which may use a CGA (color graphics adapter) mode, an EGA (enhanced graphics adapter) mode, and a VGA (video graphics array) mode, are connectable.
(2) Description of Related Art
In recent years there have developed an increasing number of computer systems which include, or permit optional connection of, several types of display controllers (e.g., a CGA, an EGA, and a VGA), so as to achieve a display function of desired performance.
Display controllers using CGA, EGA, and VGA have the characteristics identified in Table 1 below.
TABLE 1 ______________________________________ Mode Resolution Color specification ______________________________________ CGA 640*200 dots 4 colors EGA 640*350 dots 16 colors can be displayed simultaneously out of 64 colors VGA 640*480 dots 256 colors can be displayed simultaneously out of 260,000 colors ______________________________________
The display controllers include chips which achieve the above specifications and basic input/output system (BIOS) programs supporting the chips.
In such a conventional computer system, the display controller which the user wants to use is selected from among available display controllers by operating a switch. Alternatively, the display controller can be selected through an input device (e.g., a keyboard) which the user operates as he is guided through a setup program or the like after the system has been turned on. However, if the display controller to be used is a different type from the display controller used last, the user must first determine all the display setup conditions, or designate a setup program to be executed.
Some of the conventional computer systems permit an optional display controller to be connected thereto, in addition to a built-in display controller, in such a manner that the optional display adapter is located either inside or outside of the main body of the computer systems. Even if the optional display controller is located within the main body, it is usual that the built-in display controller is automatically selected prior to the optional display controller. Therefore, when the computer system is actuated the user is not allowed to freely select the display controller which he or she wants to use.